Flat panel displays, including a liquid crystal display and a plasma display, are applied to a wide range from mobile devices such as cellular phones to large sized devices such as public display devices. Many of such displays have been developed with focusing on the wider viewing angle, the higher brightness, the higher image quality, and the like, and a device has been desired which provides a clear and easy-to-view display as viewed from any angle.
On the other hand, some of the contents displayed on the display are not desired to be viewed by others, such as confidential information and private data. Now that the development of a ubiquitous information society accompanied by the development of information equipment is progressing, an important issue is that display contents are prevented from being viewed by others, even in public circumstances where unspecified people are present. In addition, confidential information not desired to be viewed by a person passing behind a seat may be handled even in an office.
Some cellular phones and the like are provided with a display that enables display contents to be viewed from only a specific direction by providing an optical shield plate (or a louver). As an improvement of this technique, as disclosed in Japanese Laid Open Patent Application No. JP-A 2006-140126, a technique is known in which a viewing angle of a liquid crystal display device is increased for a case where information to be displayed is viewed with being shared with others, whereas the viewing angle of the liquid crystal display device is decreased in the situation where the information to be displayed is not desired to be peeped by others. In the technique disclosed in Japanese Laid Open Patent Application No. JP-A 2006-140126, a louver and a transparency/scattering switching element are provided between a liquid crystal display panel and a backlight, and the louver and transparency/scattering switching element are used to switch a variable width of a viewing angle. As the transparency/scattering switching element, a PDLC (Polymer Dispersed Liquid Crystal) cell is used. Based on such a configuration, the viewing angle is decreased when the viewing angle is to be narrowed, and thereby security protection is improved.
Such techniques are, however, insufficient from the perspective of security protection, because a display can be viewed from right behind even if the viewing angle is decreased.
An image display device for solving such a problem is disclosed in Japanese Laid Open Patent Application No. JP-A Showa 63-312788. The known image display device is configured such that, when a viewer wears glasses having a picture selection function, only the person wearing the glasses is thereby allowed to recognize a secret image, whereas the others are allowed to recognize a public image.
FIG. 13 is a block diagram illustrating a configuration of the disclosed image display device. In the image display device of FIG. 13, picture data are stored in an image data storage memory 12, which has a capacity corresponding to one frame, by using an input image signal 11 under control of a frame signal 13. The image data are read twice from the image data storage memory 12 at the rate of twice of the frame frequency. The image data initially read are fed to a composing circuit 15 as a first image signal 14 compressed to ½, and the image signal subsequently read is inputted to the composing circuit 15 as a second image signal 17 with the chromaticity and brightness thereof have been converted by a chromaticity/brightness conversion circuit 16. Accordingly, the first and second image signals 14 and 17 are alternately displayed on an image display unit 18. On the other hand, shutters of the glasses 21 are driven in response to the frame signal 13 by a glass shutter timing generator circuit 19. The shutters of the glasses 21 are driven such that the image corresponding to the second image signal 17 is not viewed by the viewer. Such a configuration and an operation allows a person not wearing the spectacles to view a gray image that is a composite image of the first and second image signals 14 and 17, and not related to the first image signal, or the third image (public image), whereas the person wearing the glasses can view a desired image (secret image) corresponding to the first image signal.
Another technique for solving the above-described problem is disclosed in Japanese Laid Open Patent Application No. JP-A 2001-255844. The technique disclosed in this patent application is directed to allow only an “image-permitted” user (observer) to perceive a private image (a secret image) on the display, and a non-permitted user to simply view a random pattern, an unreadable image, or maybe a screen saver image (a public image). For this purpose, an image processing technique involving a data shielding pattern and alternating pattern is combined with a wearable device such as active glasses, which is synchronized with a display incorporating an image created by an image processing technology. According to such a technique, data can be displayed only in private on a display a public can view, on the basis of a well-known human visual capability that fuses a dissimilar image with a single image.
The inventions disclosed in the above-mentioned Japanese Laid Open Patent Publication Nos. JP-A showa 63-312788 and JP-A 2001-255844, however, suffers from a problem that, in most of liquid crystal display devices, the secret image is observed even without wearing glasses depending on the angle at which a display screen is observed.
This results from a problem that a liquid crystal display exhibits viewing angle characteristics, the gamma characteristics vary depending on the angle at which the screen is observed. In the case where the gamma characteristics vary depending on the viewing angle, the secret image is not shielded even when the brightness values of the image for shielding the secret image (hereinafter referred to as a reversed image) are set to achieve the shielding of the secret image with respect to the front direction, since the brightness values of the reversed image are different at a different observation angle.
Also, the secret image is desired to be viewable only for a specific person and shielded from the others, and therefore it is desirable to prevent as much as possible the secret image from being displayed in a direction other than that in which the specific person browses; this is not limited to the liquid crystal display device.